Radiant burner

ABSTRACT

A RADIANT BURNER COMPOSED OF A HOUSING, A GAS ORIFICE, A VENTURI, A PLENUM, A SEPARATING PLATE WITH PORTS THEREIN, A COMBUSTION CHAMBER, RADIANT ELEMENTS AND EXHAUST OPENINGS.

Feb. 9, 1971 w. H. BEST 3,561,902

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United States Patent O 3,561,902 RADIANT BURNER Willie H. Best, R0. Box868, Columbia, S.C. 29202 Filed Sept. 19, 1968, Ser. No. 760,831 Int.Cl. F23d 13/14 US. Cl. 431328 Claims ABSTRACT OF THE DISCLOSURE Aradiant burner composed of a housing, a gas orifice, a venturi, aplenum, a separating plate with ports there in, a combustion chamber,radiant elements and exhaust openings.

This invention is concerned with improving the efficiency of radiantburners. More particularly, this invention is concerned with increasingthe efliciency of radiant burners by utilizing the heat energy createdby combustions away from the emitting surface of the burner.

In the prior art there are a number of gas fired, radiant type burners.Although their burners generally employ the same basic principles, theirspecific designs vary considerably. Generally, all the prior art radiantburners depend upon surface combustion on the area of the emittingsurface. This surface when heated to a predetermined high temperaturebecomes incandescent and radiant heat rays are emitted. In order toprevent random convection air currents from cooling the radiatingsurface, generally in the prior art a nichrome wire screen is positionedin front of the emitting surface. It acts as a thermal barrier to theseconvection currents.

There are a number of disadvantages in these prior art burners. At it isnecessary to have combustion of the gas air mixture occur uniformly overthe entire surface area, it is necessary that a relatively highpercentage, 29 percent to 65 percent of the total area be in openapertures. This high percentage of openings results in the velocity ofthe gas air mixture being substantially less than the rate of flamepropagation. These apertures or openings are sufficiently small toprevent retrogression of the flame, oxidation of the gas air mixture,into the plenum even though the rate of flame propagation isconsiderably greater than the velocity of the gas air mixture throughthe apertures. This principle of flame quencing is fully disclosed inUS. Pat. 3,277,948 to Best. As a result of the relatively low velocityof the gas air mixture and the high percentage of apertures, the radiantburners of this type are highly susceptible to clogging from airbornedust particles. These particles are brought into the plenum along withthe entrained air, and the velocity of the' gas air mixture through theapertures is not suflicient to force these particles through theapertures where they would be consumed on the surface during combustion.

Anoter disadvantage of these prior art radiant burners employing onlysurface combustion is the operation and efficiency of these burners areseriously affected by low velocity air movements. The nichrome wire usedin these burners is effective as to random convection currents, but itis not effective as a shield for the surface against small airvelocities.

A further disadvantage of these prior art radiant burners is theirlimited pressure operating range. They are highly susceptible to beingoverfired with an increase in gas pressure for a fixed orifice size.Even at increased operating pressures, the gas air mixture velocity isstill substantially less than the rate of flame propagation; therefore,the flame remains in close contact with the emitting surface and thetemperature of the emitting surface will continue to increase.Obviously, this is desirable on the present type of burners; otherwise,any burning that occurred away from the emitting surface would only gen-3,561,902 Patented Feb. 9, 1971 erate convection heat which would notcontribute to additional radiation from the heater. Since the presenttype radiant burners are dependent upon the radiation from thecombustion surface, and there is no Way to recover the energy ofcombustion which occurs away from the surface, these prior art burnersare limited to a fairly narrow operating range. If a typical burner wasorificed to operate at four inches water column, the burner would beoverfired at ten or twelve inches water column and backfiring wouldoccur in the plenum.

It is therefore the primary object of this invention to provide aradiant burner which uses the energy of the exhaust gases for thepurpose of generating radiation.

Another object of this invention is to provide a radiant burner withradiation enregy being emitted from two surfaces.

A further object of this invention is to provide a radiant burner whichcan accommodate high velocities of the gas air mixture.

Still another object of this invention is to provide a radiant burnerwhich uses a small number of apertures which are of increased size.

Still a further object of this invention is to provide a radiant burnerwhich is resistant to wind interference.

Another object of this invention is to provide a radiant burner which isnot sensitive to overfiring at high pressures.

These and other objects, features and advantages of the presentinvention will be apparent from consideration of the following detailedspecification taken in conjunction with the accompanying drawings.

FIG. 1 is a partial sectional bottom plan view of the radiant burner ofthis invention;

FIG. 2 is a cross-sectional elevational view of the radiant burner;

FIGS. 3, 4, and 5 are partial cross sections of heat emitting elements;and

FIG. 6 is a partial cross section of the separating member.

Generally, the objects of this invention are achieved by providingradiation emitting elements positioned below the aperture plate.

More specifically, referring to FIG. 2, a gas orifice 10 along with aconventional venturi 11 forms an assembly which is mounted into a metalplenum 12 which forms an open mouth chamber. Plate 15 is mounted intothe plenum, supported therein by brackets 26 and insulation material 17,and forms a volume 13 to receive the gas air mixture from venturi '11.Plate 15 contains apertures or ports 16 to allow passage of the gas airmixture into the insulated combustion zone 19 which is formed belowplate 15 and above radiating heat means 21. The exact geometry of theapertures is not of importance as they may be around, square, oblong,etc. A conventional pilot light or electric ignitor, notshown, ignitesthis mixture in the combustion zone. Zone 19 is completely insulated onall of the sides at 17 and the top by plate 15. The insulation materialand plate 15 are composed of Fibrofax, manufactured by the CarborundumCompany, which is ceramic fibers pressed into a flexible sheetcomposition. It has low thermal conductivity, excellent resistance tothermal shock and an extremely low coeflicient of expansion. Due tothese properties, the apertures can be placed in the sheet compositionafter it has been formed. This is an excellent advantage over thefragile ceramic material generally used in the radiant burners of theprior art as the apertures or ports must be formed in the fragileceramic as it is manufactured. Thus, the energy created by the flame inthe combustion zone is all concentrated against radiant elements 21which are supported in the plenum by a frame 20 which is supported bybrackets 23.

3 Frame 20 is of conventional design for example two support surfaces,one on each side of said plenum, at the open end.

Radiant heat emitting elements 21 are fabricated out of various types ofstainless steel, or a nickel chromium alloy, depending upon the desiredtemperature limits which may vary from as low as around 1,000 F. to ashigh as 2, 100 F. and above, depending upon the desired use of theradiant burner. These elements are positioned below and as close to theplate 15 as around about one half inch to around about three inches. Theelements may vary in height from around about one fourth of an inch toaround about two inches. They can be of many shapes as shown, forexample, in FIGS. 3, 4 and 5. The exact element geometry is not ofimportance. As shown in FIG. 2, these elements are positioned on clipsor brackets which created a partially enclosed area 25 in the bottom ofthe plenum below the elements. There are openings 22 between and amongthe emitting elements so the products of combustion which occurs in zone19 will exit the combustion zone; therefore, adding their heat energy tothe radiating elements. Thus, with the burner of this invention, notonly the heat energy from the combustion zone is given to the radiatingelements, but the heat energy of the combustion products themselves areadded to the radiating elements thereby greatly increasing theefliciency of this burner over the burners of the prior art.

Gas is discharged through orifice and primary air for combusion isentrained at thi point and the gas air mixture is discharged into volume13. This mixture of gas and air then passes through apertures 16 intocombustion zone 19 where it is ignited by a conventional pilot light.The combustion or burning of this mixture is contained within the zone19. The total aperture area of plate 15, somewhat less than 29 percent,is such that the velocity of the mixture passing through the aperturesis greater than the rate of flame propagation, thus removing the problemof flame retrogressing into volume 13. Plate is present in the radiantburner to provide a base for the apertures as Well as dividing the gasair mixture volume from the combustion chamber, and also insulation forthe top of the combustion zone. As it is fabricated from a material oflow thermal conductivity, it prevents the heat energy produced in thecombustion zone from escaping into volume area 13 where it would bewasted. Thus, it is not of importance that the com bustion of the gasair mixture does not occur on the surface of plate '15 as is necessaryin the radiant burners of the prior art. The flow of hte heat energy ofcombustion zone 19 is restricted by the insulation properties of plate15 and the side insulation 17 and channeled to flow only downwardly toradiation emitting elements 21.

The radiant burner of this invention is so designed that one hundredpercent of the air necessary for complete combustion is available inzone 19 without the requirement of secondary air. Combustion occurs inthis zone or chamber and all of the internal surfaces are heated to ahigh temperature, from l,000 F. to around 2,100 F., depending uponoperating requirements. The radiating elements 21 are heated by theirinternal surfaces being directly exposed to the combustion zone and byabsorption of radiation from this zone. The products of combustionpassing through openings 22 also add heat to the radiating elements. Asthe combustion is confined in zone 19, it is not necessary to haveintimate flame contact with plate 15 in order for it to be heated as inconventional burners. Obviously, all of the internal surfaces ofcombustion zone 19 are exposed to the high temperatures of combustionand to the high temperatures from the combustion products. The entirezone or area is filled with products of combustion approaching the flametem perature as the products of combustion have not been diluted byexcess air. Thus, all the internal surfaces of the combustion zone areheated to a high temperature; therefore, heat energy will be emittedfrom plate separator 15 as Well as the heat emitting elements. This isadvantageous as the burner of this invention will emit heat energy fromtwo surfaces probably of difierent temperatures.

This gives a wider range of heat.

The gas air mixtures velocity through the apertures 16 be much higherthan in the conventional burners where surface combustion is essential.Due to this increased velocity, the size of the individual apertures canbe much larger than where retrogression is prevented by the quenchingphenomena. Thus, due to the combination of large apertures and highvelocities, the radiant burner of this invention almost totallyeliminates the problem of clogging due to dust particles entrained inthe air. These particles pass through the large apertures and areconsumed in the combustion zone. In addition, due to the design of theradiant burner of this invention, it may operate under pressure as lowas 3 inches water column gas pressure and as high as 40 inches watercolumn gas pressure and above. These pressures with the proper burneraperture area will cause a gas air mixture velocity to be greater thanthe rate of flame propagation thereby preventing retrogression of theflame into volume 13.

The design of the radiant burner of this invention provides forconsiderable thermal inertia, retention of heat, in the radiatingelements. This combined with the fact that the combustion zone ispartially protected by the radiation elements results in the burnerbeing resistant to Wind disturbances than the conventional burnersemploying surface combustion. In addition, where wind is prevalent,there is a distinct advantage in having high thermal inertia in theradiating elements to prevent rapid cooling under gusty or momentarilyhigh velocity Wind conditions. Further, as the combustion is confined inzone 19, considerable radiation can be emitted from both the flame andthe products of combustion. The products of combustion are confined in athick layer in the combustion zone by the geometry of the burner,whereas, in conventional burners, they are rapidly expelled from thesurface in a very thin layer. After these products of combustion,exhaust gases, pass through openings 22 in the radiation elementassembly, the elements then become protected against cooling from freeconvention by the hot layer of exhaust gases contained under them inzone 25. If free convention currents cannot readily reach the radiatingelements, the energy in these elements can then only escape byradiation. Consequently, high radiation efficiencies are achived in theradiant burner of this invention.

In FIG. 5, the radiation elements are so arranged that all radiation isomitted from these elements. There is only an indirect opening into thecombustion zone and the burner is thus given additional protectionagainst wind. The emitting elements may also be coated with varioussubstances, for example a high temperature liquid ceramic that willefiect the quality of radiation in special applications where this maybe an advantage.

The operating versatility olfers tremendous advantages in processapplication. This versatility allows the radiant burner of thisinvention to operate over a wide range of temperatures, for example fromaround 1,000 P. to around 2,100 F. The design of the radiant burner ofthis invention represents the first departure from surface combustionfor this type of radiating device. It is believed that the hightemperature achieved on the internal surfaces of combustion zone 19 isdue to both the flame contact and the high temperature of the productsof combustion. The pressure in combustion zone 19 can be controlled byvarying the total area of openings 22. Obviously, as the openingsincrease in size, the pressure in zone 19 will decrease. The gas airmixture velocity through apertures 16 is dependent on the difference inthe pressure of zone 13 and 19. Since the pressure of zone 19 can becontrolled by the openings 22, an excellent method is provided forbalancing the pressures throughout the entire system. In addition, theheat radiating elements of this invention are so constructed andarranged as to emit a substantial portion of the total heat emitted fromthe burner, with the small balance coming from he separation member.

It will be obvious to those skilled in the art that many variations andmodifications may be made in the above without departing from the truescope and spirit of the invention as defined in the appended claims.

I claim:

1. A radiant burner including an open faced housing defining a plenum, aperforated separating member in the form of a flat plate dividing theplenum into a fuel supply area and a combustion area, the area of theperforations in the said member forming less than twenty nine percent ofthe area of its perforated surface, a heat radiating means extendingacross the combustion area and spaced from the separating member by adistance substantially greater than the thickness of that member, theheat radiating means having a thickness at least as great as that of theseparating member and comprising a pair of spaced support members and aseries of equally spaced parallel heat emitting elements extendingbetween said support members, and means to supply a combustible mixtureof air and fluid fuel under pressure to the fuel supply area, the meansfor supplying air and fuel supplying them at such pressure that theywill pass through the openings in the separating member at a velocitygreater than the rate of flame propagation so that the flame will bespaced from the surface of said separating member; and the heat emittingelements being generally V-shaped but with one leg of the V extendinghigher than the other and bent outwardly at its upper portion, theelements being so arranged that the bent portion of each one extendsover the shorter leg of the adjacent element to provide an outletpassage; thus providing a circuitous passage for protection againstwind.

2. A radiant burner including an open faced housing defining a plenum, aperforated separating member in the form of a flat plate dividing theplenum into a fuel supply area and a combustion area, the area of theperforations in the said member forming less than twenty nine percent ofthe area of its perforated surface, a heat radiating means extendingacross the combustion area and spaced from the separating member by adistance substantially greater than the thickness of that member, theheat radiating means having a thickness at least as great as that of theseparating member and comprising a pair of spaced support members and aseries of equally spaced parallel heating emitting elements extendingbetween said support members, and means to supply a combustible mixtureof air and fluid fuel under pressure to the fuel supply area, the meansfor supplying air and fuel supplying them at such pressure that theywill pass through the openings in the separating member at a velocitygreater than the rate of flame propagation so that the flame will bespaced from the surface of said separating member; the perforations inthe separating member being of uniform shape and area throughout theirextent and being of sufficient size to prevent clogging by dust or otherparticles.

3. A radiant burner including an open faced housing defining a plenum, aperforated separating member in the form of a flat plate dividing theplenum into a fuel supply area and a combustion area, the area of theperforations in the said member forming less than twentynine percent ofthe area of its perforated surface, a heat radiating means extendingacross the combustion area and spaced from the separating member by adistance substantially greater than the thickness of that member, theheat radiating means having a thickness at least as great as that of theseparating member and comprising a pair of spaced support members and aseries of equally spaced parallel heat emitting elements extendingbetween said support members, and means to supply a combustible mixtureof air and fluid fuel under pressure to the fuel supply area, the meansfor supplying air and fuel supplying them at such pressure that theywill pass through the openings in the separating member at a velocitygreater than the rate of flame propagation so that the flame will bespaced from the surface of said separating member, the heat emittingelements being in the form of V-shaped bars of stainless steel coatedwith a high temperature liquid ceramic.

4. A radiant burner including an open faced housing defining a plenum, aperforated separating member in the form of a flat plate dividing theplenum into a fuel supply area and a combustion area, the area of theperforations in the said member forming less than twenty-nine percent ofthe area of its perforated surface, a heat radiating means extendingacross the combustion area and spaced from the separating member by adistance substantially greater than the thickness of that member, theheat radiating means having a thickness at least as great as that of theseparating member and comprising a pair of spaced support members and aseries of equally spaced parallel heat emitting elements extendingbetween said support members, and means to supply a combustible mixtureof air and fluid fuel under pressure to the fuel supply area, the meansfor supplying air and fuel supplying them at such pressure that theywill pass through the openings in the separating member at a velocitygreater than the rate of flame propagation so that the flame will bespaced from the surface of said separating member; the perforations inthe separating member being in the form of slots extending parallel toeach other, and the heat emitting elements of the heat radiating memberbeing in the form of generally V-shaped bars extending in a direction atright angles to the longitudinal dimension of said slots.

5. A radiant burner including an open faced housing defining a plenum, aperforated flat separating member dividing the plenum into a fuel supplyarea and a combustion area, the perforations being in the form ofparallel slots of uniform cross sectional area throughout and ofsufiicient area to prevent clogging by dust, the combined area of theperforations being less than twenty-nine percent of the area of themember, a heat radiating means of a thickness at least as great as thatof the separating member and spaced from the member by a distance asgreat as the thickness of the member, to form the combustion area, arefractory lining for the walls of the housing between said separatingmember and heating I element, the separating member being supported bysuch lining, the heat radiating means comprising a pair of spacedsupport members and a series of equally spaced elongated parallel heatemitting elements extending between said support members in a directionat right angles to the greater dimension of the slots in the separatingmembers, said heat emitting elements being formed of metal coated with ahigh temperature liquid ceramic and being generally V-shaped in crosssection but with one leg of the V extending higher than the other andbent outwardly at its upper portion, the elements being so arranged thatthe bent portion of each one extends over the shorter leg of theadjacent element to provide an outlet portion, thus forming a circuitousoutlet passage for protection against wind, and means for supplying acombustible mixture of gas and air to the fuel supply area under suchpressure that it will pass through the openings in the separating memberat a velocity greater than that of flame propagation, thus causing theflame to be spaced from the said member and close to the heat radiatingmeans.

References Cited UNITED STATES PATENTS 3,445,175 5/1969 Krieger 43l--328 3,258,058 6/ 1966 Lherault 431--329 FOREIGN PATENTS 905,481 9/1962 Great Britain 431328 EDWARD G. FAVORS, Primary Examiner

